FIG. 1 is a cross-sectional view of a prior art tandem solar cell comprising an a-Si:H (hydrogenated amorphous silicon) n-i-p solar cell serially arranged on an a-Si:H/c-Si (crystalline silicon) solar cell disclosed in "Proceedings Of The 2nd International Photovoltaic Science And Engineering Conference", 1986, pages 394-397. A p-type crystalline silicon or polycrystalline silicon (poly-Si) substrate 1 has serially disposed on it an n-type hydrogenated amorphous silicon layer 3, a p-type a-Si:H layer 7, an intrinsic a-Si:H layer 8, an n-type a-Si:H layer 3a, and a transparent electrode layer 4. Grid electrodes 5 are selectively disposed on the transparent electrode layer 4 and a rear surface electrode layer 6 is disposed on the rear surface of the substrate 1.
The n-type a-Si:H layer 3, p-type a-Si:H layer 7, intrinsic a-Si:H layer 8, and n-type a-Si:H layer 3a are successively deposited on the p-type c-Si or poly-Si substrate 1 in a plasma chemical vapor deposition (CVD) process. The tandem structure includes an n-i-p solar cell comprising a-Si:H layers 3a, 8, and 7 serially connected with an n-type a-Si:H/p-type c-Si or poly-Si solar cell. An ITO (indium tin oxide) transparent electrode layer 4 is deposited on the n-type a-Si:H layer 3a by sputtering or evaporation. The grid electrodes 5, comprising silver and having a spacing of several centimeters, are printed on the transparent electrode layer 4. The rear surface electrode layer 6 is formed by sintering an aluminum paste applied to the rear surface of the substrate 1 before the a-Si:H layers are deposited.
In this tandem solar cell, light of relatively short wavelengths within the solar spectrum is converted into electricity by the n-i-p solar cell comprising the a-Si:H layers, and light that passes through the n-i-p solar cell is converted into electricity by the n-type a-Si:H/p-type c-Si or poly-Si solar cell. Thus, a wide range of the solar spectrum is utilized to generate electricity in the tandem solar cell.
FIG. 2 is a cross-sectional view of a prior art silicon microcrystalline (.mu.c-Si:H) or a-Si:H/c-Si or poly-Si solar cell. An n-type c-Si or poly-Si substrate of 400 to 500 microns thickness has deposited on it a p-type .mu.c-Si:H layer or an a-Si:H layer 3 100 to 200 angstroms thick and a transparent electrode layer 4. A comb-type electrode 5 is disposed on the transparent electrode 4 and a rear surface electrode 6 comprising silver 6,000 angstroms thick is disposed on the rear surface of the substrate 1.
The p-type .mu.c-Si:H layer or a-Si:H layer 3 is deposited on an n-type c-Si or poly-Si substrate 1 in a plasma CVD process and the transparent electrode layer 4 is deposited by sputtering or evaporation. The rear surface electrode 6 is deposited on the rear surface of the substrate 1 and the grid electrodes 5 are deposited on the transparent electrode 4 by printing or evaporation.
In this .mu.c-Si:H or a-Si:H/c-Si or poly-Si solar cell, a pn junction is present between the n-type c-Si or poly-Si substrate 1 and the p-type .mu.c-Si:H layer 3. When sunlight is incident on the device, holes are produced in the substrate 1 and collected as a result of the potential gradient of the pn junction, thereby generating electricity.
FIG. 3 is a cross-sectional view of a MIS-type solar cell disclosed in "15th IEEE Photovoltaic Specialists Conference", 1981, pages 1405-1408. A p-type c-Si or poly-Si substrate 1 bears an oxide film 2. A collecting electrode layer 5 is disposed on the oxide film 2. A rear surface electrode layer 6 is disposed on the rear surface of the substrate 1.
The oxide film 2 is formed on the c-Si or poly-Si substrate 1 by thermal oxidation. An aluminum layer is deposited on the oxide film 2 by evaporation or sputtering and patterned by photolithography into a grid configuration having a spacing between grid fingers of several microns several tens of microns. The rear surface electrode 6 is formed by sintering aluminum paste which is deposited on the rear surface of the substrate before the oxide film 2 and the electrode layer 5 are formed. Metal M of the grid electrode pattern with the thin oxide film I and the c-Si or poly-Si substrate S comprise an MIS structure that generates electricity from light absorbed by the Si substrate. The range of the solar spectrum absorbed by the MIS structure is narrower than the range absorbed by the tandem structure.
In the prior art a-Si/c-Si or poly-Si solar cell constructed as described above, no matter how the c-Si or poly-Si substrate is cleaned, an interface state density of about 10.sup.9 eV.sup.-1 cm.sup.-2 is present at the a-Si:H layer/c-Si or poly-Si junction interface, limiting the open circuit voltage V.sub.oc. Furthermore, the open circuit voltage and the fill factor, i.e., the product of the current and the voltage corresponding to the optimum operation point of the solar cell divided by the product of the open circuit voltage V.sub.oc and the short circuit current J.sub.sc, are lowered because the a-Si:H layer/c-Si or poly-Si interface is affected by the plasma in the plasma CVD process, increasing the interface state density. In addition, optical and electrical deterioration, such as the Staebler-Wronski effect, occurs, especially in the neighborhood of the interface, when light irradiates the cell for a long time.
There is no established theory explaining the cause of the degradation of cell performance. However, the following theory is advanced. Generally, in a-Si:H and .mu.c-Si:H, silicon bonds which are not compensated by hydrogen are so-called "dangling" bonds. Since no atom is present at the end of a dangling bond, traps occur, adversely affecting the electrical characteristics of the non-crystalline silicon. In order to compensate the dangling bonds, hydrogen is added to the non-crystalline silicon and couples with the dangling bonds. However, when light irradiates the silicon, the hydrogen-silicon bonds are broken, resulting in degradation of the solar cell characteristics.
When an a-Si:H n-i-p solar cell is disposed on the a-Si:H/c-Si or poly-Si solar cell, in the tandem solar cell of FIG. 1, the same current must flow through both cells. Since the upper a-Si:H n-i-p solar cell has a smaller current output than the lower solar cell, the total output current is limited by the upper n-i-p solar cell. In the prior art .mu.c-Si:H/c-Si or poly-Si solar cell, the solar cell characteristics, such as open circuit voltage, short-circuit current, fill factor, and the like, are not always good. In the prior art MIS solar cell, to make sufficient light incident on the c-Si or poly-Si substrate, patterning of a fine metal electrode having a width of 5 to 15 microns and a spacing interval of 50 to 120 microns is required. That patterning requires an expensive process, such as photolithography.